Cannabinoid CB 1 receptor-mediated modulation of evoked dopamine release and of adenylyl cyclase activity in the human neocortex

Abstract

1 The present study investigated the binding characteristics of various ligands to cannabinoid CB 1 receptors in human neocortex and amygdala. In addition, the functionality of CB 1 receptors in the human neocortex was assessed by examining the effects of CB 1 receptor ligands on evoked [ 3H]-dopamine (DA) release in superfused brain slices and on synaptosomal cAMP accumulation. 2 Saturation-binding assays in human neocortical and amygdala synaptosomes using a radiolabelled cannabinoid receptor agonist ([ 3H]-CP55.940) revealed pK d values of 8.96 and 8.63, respectively. The numbers of binding sites (B max) were 3.99 and 2.67 pmol (mg protein) -1, respectively. 3 Various cannabinoid receptor ligands inhibited [ 3H]-CP55.940 binding with rank order potencies corresponding to those of previous studies in animal tissues. 4 Electrically evoked [ 3H]-DA release from human neocortical slices was inhibited by CP55.940 (IC 50 6.76 nM, I max, 65%) and strongly enhanced by the cannabinoid receptor antagonist AM251. However, [ 3H]-DA release was not influenced in rat neocortex. In human tissue, the estimated endocannabinoid concentration in the biophase of the release-modulating CB 1 receptors was 1.07 nM, expressed in CP55.940 units. 5 K +-evoked [ 3H]-DA release in the presence of tetrodotoxin (TTX) was strongly inhibited by CP55.940 in humans, but not in rats. 6 In human tissue, CP55.940 inhibited forskolin-stimulated cAMP accumulation (IC 50 20.89 nM, I max 35%). AM251 blocked this effect and per se increased forskolin-stimulated cAMP accumulation by ∼20%. 7 In conclusion, cannabinoids modulate [ 3H]-DA release and adenylyl cyclase activity in the human neocortex. CB 1 receptors are located on dopaminergic nerve terminals and seem to be tonically activated by endocannabinoids.